1. Field of the Invention
This invention relates to computer displays and more particularly to a method and system for achieving accurate white point settings in computer CRT displays by measuring and controlling the CRT beam current.
2. Description of the Background Art
Calibrating and maintaining accurate color settings on a computer CRT is a problem which is increasingly drawing attention of computer users as display technology and requirements advance. Although color CRT technology has improved dramatically over the past two decades, the increased availability of inexpensive digital video components has created a user demand for more exacting color reproduction in even the lowest cost computing systems.
This demand for precision color has resulted in the introduction of a number of elaborate devices and techniques for calibrating and adjusting the CRT displays of computing systems. Several products are currently available for measuring and providing highly accurate values of the luminance and hue of CRT displays. With such a device the user can display an image having known color characteristics, and manually calibrate the CRT display to produce the known characteristics of the image on the display screen or can manually initiate automatic calibration using an external light sensor.
The disadvantage of this manual calibration technique is that the measurement must be periodically made to the CRT, as the characteristics of the display change over time. Each of the three guns (one corresponding to each primary color, usually Red, Green, and Blue) will change over time as the cathodes wear and erode. The mechanisms of wear relate to changes in the amount of beam current resulting from the voltage drive from the computer and also to changes in the voltage from the computer that corresponds to no beam current (also referred to as "black level"), these are known as changes in emission and cutoff, respectively. These characteristics also change with temperature, notably for the first few minutes after a display is turned on and until it "warms up", but this warm-up drift often continues at a substantial rate for up to several hours after initial turn-on.
Other aging occurs in the screen phosphors. The phosphors become less efficient with time. The decrease in light output for a given beam power input is proportional to the total amount of energy delivered to the phosphor screen over the life of the screen. Fortunately, in many color displays, if the beam currents for the primary colors are kept in constant proportions, the hue of the emitted light will remain the same in spite of decreasing luminance.
Another wear mechanism is referred to as glass browning, which is manifested as loss of blue transmission. It is attributed to the metalization of the sodium ions in the glass that makes up the CRT envelope and is due to x-ray bombardment resulting from the impact of high-energy electrons, over several years of operation. With modern CRT glass formulations, sodium in the ionized state is relatively sparse, and browning is not a severe problem. It does, however, contribute measurably to color shift after many years of operation. Although the efficiency of screen phosphors changes over time, the light output continues to be a fairly linear function of input power, and thus input current. As noted previously, the phosphors for each primary color generally age fairly uniformly such that if a given current ratio is maintained over the life of the CRT, the color of light emitted will not vary much. A typical example is the 14" Trinitron (.TM. Sony Corp.) color monitor used by Apple Computer. If beam currents are maintained at luminance levels above the maximum shipped brightness, the hue drift would amount to about 2.7 .DELTA.E* units over the first 2,000 hours (1 .DELTA.E* is similar to a just-noticeable difference in color ).
Since the emission and cutoff of the cathodes age continuously, the operating points of the tube must constantly be re-adjusted to maintain precise color rendition. What is needed is a system for measuring the current to each CRT cathode and for adjusting the voltage signal delivered to the cathode from the CPU to obtain the desired current, and therefore the desired luminance from each primary color to ultimately obtain the desired hue and luminance.